Electron device and method of activation



LPatented May 22,

v g 4 1,670,483 uru'rlazn STATES PATENT OFFICE.

mm BENTSCKLEB, 01' EAST ORANGE, NEW JERSEY, ABSIGIIOB 1'0- WESTINGHOUSE LAD OOIIPANY, A CORPORATION 01 PENNSYLVANIA.

ELECTRON DEVICE AND IETHOD OI ACTIVATION.

Io Drawing. Application fled Kay 4,

This invention relates to electron devices and material adapted to be used for electronemission purposes. and more particularly to the employment of a compound of a radioactive element of group II of the eriodic table of elements, for increasing t e electron-emissivity of such material.

object-of my invention is the activation of r'efractor metallic filament or ribbon by means 0 a compound. of a radioactive substance in group II, for example, radium oxide or the like, applied as a coating thereto.

Another object of my invention is the production of filament or ribbon of such material as latinum-iridium or the like, to which is rml aflixed an even coating of a compound 0 radium or mesothorium, whereby the electron-emissivity thereof is increased and caused to evidence itself at lower temperatures than other usual electron-emission material.

A further object of my invention is the manufacture of platinum-iridium filmament or ribbon, rendered highly sensitive for electron-emission purposes by the application thereto of a coating of a substance such as radium oxide, mesothorium oxide or the like, either alone or mixed with alkaline earths; for example, barium oxide or strontium-oxide which is so efficient that only a small proportion need be applied and which shows the requisite emission at a lower temperature than other electron-emission material generally used.

Other objects and advantages will be disclosed upon reading the following description.

As is well know, one of the usual methods of'activating filamentary material for electron-emission purposes 1s to applya coating thereto of certain oxides or mixtures thereof. Substances which have been used are the oxides of calcium, strontium and barium, which are generally known as the alkaline earths.

Several methods have been proposed for applying such an oxide-coating to wire or filamentary material .for electron-emission pur oses. The filamentary material used is pre erably platinum-iridium wire or ribbon,

as such material does not react with the oxides upon heating.

H. D. Arnold, in his paper entitled Phenomena in oxide-coated filament electron 1988- Serial llo. 838,765.

11, 1922, electron emitting devices and method of making and assigned to the Westinghouse Lamp Company, is described a I preferred method of coating platinum-iridium ribbon with carbonates of alkaline earth metals without the use of an organic binder and thereby accomplishing the same result obtained by Arnold without necessitating the introduction of an or anic binder, which has to be subsequently e iminated b evaporation when in an electron tube an thereby obviating-the introduction of deleterious matter into such a tube.

Although the employment of alkaline earths for electron-emission purposes has given good results, it has been found preferable to use a relatively heavy coating thereof on the filamentary material to accomplish the desired results and such coating is, therefore, all the more liable to become detached therefrom when in service. Although the requisite emission is obtained by the use 'of alkaline earths at lower temperatures than when thorium is used for the same purpose, yet the temperature at which efi'ective electron-emission occurs is not as low as might be desired.

According to my invention, I appply a coating of a compound of one of the radioactive elements of group II, of the periodic system of the elements, and preferably apply such coating in a manner similar to that disclosed for applying alkaline-earth coatin s to filamentary material. Briefly, my met od may comprise forming a suspension of oxide or other com ound of one of the radioactive elements 0 group II, for example, radium, and applying such suspension to the platinum-iridium ribbon or other filamentary material used. The ribbon or wire may then be heated electrically to dry and bake the coating thereon to cause it to firmly adhere thereto.

The filamentary material so treated may then be employed as the electron-emitting element or hot cathode in an electron tube,

where it will be found to efficiently emit electrons at a lower temperature than that to which it is necessary to heat alkalineearths and thorium for the. purpose and it s only necessary to employ a very thin coating or film of such substance on the filamentary material to accomplish such result. Although the coating of the filamentmay be produced by dipping in the suspension only once, a larger uantity may be applied 1n a more firmly ad erent manner, thereby giving better activation, by dipping or passing the filament through the suspension, or otherwise coating it, a plurality of times and subjecting the same between each dipping to a heating or drying process for the purpose of causing the coating to firmly. adhere thereto.

My invention will better be understood by the detailed description which follows. Although the substances proposed for activating filamentary material according to my invention, are relatively expensive at the present time, the cost of activation by such material would not be excessive, because only very minute quantities thereof are necessary for efficiently activating the material. The radio-active elements in group 11 comrise radium and mesothorium, of which the ormer is preferred as obtained in a pure state. Although other compounds may be employed, radium oxide is preferred and is the substance, the activation by which will be specifically described.

A suspension of radium oxide may be prepared by making a paste of a small quantity thereof with water and thinning out to the desired consistency, which may be very dilute, as a quantity applied to a platinumiridium ribbon, so small that it could hardly be detected with a microscope, has been found to give good activation at a relatively low temperature. The wire or ribbon, preferably formed of platinum-iridium, may then be coated with the suspension, as by passing it over or around a gettering wheel dipping therein, or in any other desired manner.

Although a suspension has been mentioned in connection with a preferred manner of applying the radium oxide to the ribbon or filament, it is obvious that the same may be applied with the aid of a suitable binder vehicle such as nitro-cellulose dissolved in amyl acetate. The plain water suspension is preferred, because it obviates the introduction of deleterious organic matter into the electron devices in which the filament may be employed.

The filamentary material, after being coated with the suspension may be passed through an electric furnace where the coating is dried and the solid material firmly baked on. If a thicker coating on the filamentary material is desired, the application being more readily of the suspension and subs uent heating of the material may be repeate Such a process of coating a filament is fully disclosed in the copending, application of MacRae, Serial No. 587,270, before referred to, and 7. the filament may be coated with radium oxide or the like in the same manner as there disclosed for coating a filament with an alkaline-earth carbonate or mixture of carbonates, except that it is not necessar to maintain an atmosphere of carbon dioxi e in the electric furnace while the baking process is being carried on.

A'ribbon or filament of platinum-iridium so treated was mounted as the cathode on a regular mount for athree-electrode or radio tube, provided with a regular molybdenum grid and plate and magnesium and aluminum welded to the plate for cleaning-up small traces of residual gases.

The size of platinium-iridium ribbon used for performing the following described experiments, was 5.2 cms. in length .0021 cms. wide and weighed about 7.23 milligrams per 200 millimeters. Such a mount was then sealed into a regular tube therefor, and connected to an all-glass system with a liquid-air trap and mercury vapor pump.

After baking-out the tube on the ump at about 400 C. the clean-up meta was va orized by the bulb by heating the plate. W en the pump and the clean-up metal had brought the pressure of thegas therein to about .01 of a micron of mercury, the filament was lighted and 60 volts put on the late and id for a comparative test of the lament. a v'hen the current through the filament had reached .15 ampere the plate current was .1 milliampere. n slowly raising the filament current, the plate current became more than 10 milliamperes at .3 ampere filament current. In testing another tube similarly constructed, .1 milliampere plate current was obtained when the filament current was .1 ampere. The plate current rose to 5 milliamperes at .157 ampere of filament current. After sealing-off the tube, measurements at .15 amperes filament current, give 2.3 milliamperes plate current and at .2 am ere filament current, 36 milliamperes of p ate current was obtained. The grid current was then .1 milliampere. The amount obtained therefrom, and before described, is higher than that obtained when such a filament was coated with barium and strontium oxides, heated to the same temperature and mounted in the same way in a similar bulb.

On comparingresults, it was observed that the emission started at a lower temperature when the radium oxide coated filaments were tested, than when oxides of barium and strontium were employed and it should be noted that the amount of radium oxide a plied thereon was so small, that it co d Ill - amuse barely be detected with a microscope. For that reason, activation with such material would not be prohibitive, although compounds ofradium and mesothorium are very expensive.

Altho h I have described a preferred method 0 practicing my invention, it is to be understood that the same is merely illustrative and that modifications within the s irit and scope of the ap nded claims, and

erefore to be considered as my invention, ma occur to those skilled in the art.

at is claimed is:

1. The method of increasing the thermionic activity of filamentary material comprising applying thereto a coating of radium on e.

2. The method of activating material for thermionic purposes :frlsl making a suspension of wde r um oxide,

coating the material therewith and heating the same to aflix it thereto.

3. Refractory filamentary material for coated wit an oxi e of radium.

5. An electron tube com rising an evacuated envelope enclosing a p ate, a grid and a refractory metal filament activated by a coatin of oxide of radium.

6. 'i im method of coating cathode which comprises wettin the cathode with a liquid containing a ra oactive material distributed uniformly throughout the liquid in an attenuated state and removing the liquid from the radioactive material whereby a uniform and adherent coating of the radioactive material over the surface of the cathode is obtained.

In testimony whereof, I have hereunto subscribed my name this 3rd day of May, 1923.

HARVEY CLAYTON IEI'I'SCIILER.

a thermionic 

